Fourthly,
concerning the dust abundance,
the simulations of [1]} or [2]} find depletions
by a factor of
\(10^3\) –\(10^4\)  relative to the global disc abundance.
In a sample of seven discs, [3]} found with a method independent of a tracer-to-mass conversion a globalAlthough \(f_{\textrm {d/g}}\)  likely varies on global disc scales [4]}.
dust-to-gas ratio of order \(f_{\textrm {d/g}}\sim 10^{-4}\) –\(10^{-3}\) .
Recent global-disc simulations of grain growth and drift (e.g. [5]}, [6]}) lend support to this.
Finally, if planets form early (e.g. [7]}), the dust particles may be locked up in macroscopic objects (pebbles, planetesimals, planetary cores), also reducing \(f_{\textrm {d/g}}\) .
